Jeff is right..you can't have too much O2 . I make sure to aerate my water at every point it leaves a pipe by drilling 5/32 holes on a down angle to create a venturi effect. Its worked well for 4 years. You just have to go around with a pipe cleaner about one a month and brush out the holes as gunk/algae builds up. This and a good air pump are keeping my fish happy.

I agree with Jeff you cant have too much O2. I borrowed an O2 meter (YSI 550A) from a local school and had a little fun checking different areas of my system, I found that the areas that I thought might be low in O2 where fine,glad I didn't spend the money on a meter.